The present disclosures generally relate to integrated circuits, and more particularly, to an integrated fault detector circuit and method for detecting a fault.
Existing fault detector circuits utilize voltage monitoring as the principal method of detecting open load conditions. Such circuits utilize a voltage comparator and a floating current source, each connected across the drain and source of a high-current switch transistor. When the high-current switch transistor is in the OFF state and the load is connected properly, the voltage across the high-current switch transistor will be equal to the supply voltage. However, if the load becomes disconnected, the small amount of current flowing in the floating current source will cause the differential voltage of the high-current switch transistor to collapse. Such a collapse of the differential voltage is detected by the voltage comparator and reported as an open-load fault.
In a known fault detection circuit 10, such as shown in FIG. 1, a high current switch 12 is connected in a low-side configuration. High current switch transistor 12 can comprise, for example, an n-channel MOSFET and includes source 14, drain 16 and gate 18 terminals. The fault detector circuit 10 of FIG. 1 uses floating current source transistors M3 and M9, indicated by reference numerals 20 and 22, respectively. The floating current source provides a controlled current from the drain to source of the MOSFET device. As noted above, if the load becomes disconnected, the small amount of current flowing in the floating current source will cause the voltage from drain to source of the high-current switch transistor 12 to collapse. In addition, the fault detector circuit 10 uses voltage divider resistors R1 and R2, generally indicated by reference numeral 24, to attenuate the drain voltage (sns_DRAIN) of high current switch transistor 12 to the positive input 26 of the comparator (COMP1), indicated by reference numeral 28. The fault detector circuit 10 further includes voltage divider resistors R3 and R4, generally indicated by reference numeral 30, to attenuate the source voltage (sns_SOURCE) of high current switch transistor 12 to the negative input 32 of the comparator 28. Comparator 28 determines when the voltage between the drain 16 and the source 14 is (i) greater than or (ii) less than a predetermined threshold voltage, for example, Vds>3V. When the high current switch 12 is OFF and the combined small current flowing in floating current source transistor (M9) 22 plus the small current flowing in R1 and R2 is sufficient to pull the drain voltage below its predetermined threshold voltage, the comparator output (OUTPUT) 34 is driven low. The circuit 10 of FIG. 1 is configured for a low-side drive application, wherein the load (RLOAD) 60 is connected between a battery (VBATT) 62 and the drain terminal 16 of the high current switch transistor 12. To complete the circuit, the source 14 of the high current switch transistor 12 is connected to VSS. A low voltage on the comparator output 34 indicates that the load (RLOAD) 60, which should be connected to the high current switch transistor 12, is absent.
Circuit 10 still further comprises transistors (M4,M6) 40 and 42, current source (I1) 44, and transistors (M7,M8) 46 and 48. Voltage VCP is indicated by reference numeral 50, voltage V5 is indicated by reference numeral 52, and voltage VSS is indicated by reference numeral 54.
Further with respect to FIG. 1, VCP is a power supply voltage, indicated by reference numeral 50, which must exceed the maximum required voltage at the source of the high current switch transistor (when the transistor is utilized in a high side drive configuration). In the high side drive configuration, the load resistor is connected between the source terminal of the high current switch transistor and VSS and the drain of the high current switch transistor is connected to VBATT. The reason that VCP must be greater than the source voltage is to provide the required operating voltage for transistors M3, M9, M4 and M6, as indicated by reference numerals 20, 22, 40 and 42, respectively.
V5 is a power supply voltage required to power current source I1 and mirror transistors M7 and M8, as indicated by reference numerals 44, 46 and 48, respectively. In addition, VSS is another power supply voltage, indicated by reference numeral 54, such as a negative supply voltage required to power current source I1 and mirror transistors M7 and M8. VSS also provides a reference potential for resistors R1, R2, R3 and R4, as indicated collectively by reference numerals 24 and 30. VSS can comprise a negative power supply voltage, zero volts, or a suitable voltage other than zero volts, as may be required for a given circuit implementation. In the low side drive configuration pictured in FIG. 1, VSS also powers transistors M3, M9 and the high current switch transistor, indicated by reference numerals 20, 22 and 12, respectively.
Current source I1, indicated by reference numeral 44, comprises a current source which those skilled in the art of circuit design recognize may be realized in many different ways such as a single resistor or combinations of transistors and resistors. Transistors M4 and M6, indicated by reference numerals 40 and 42, respectively, comprise p-channel MOSFET transistors connected as a current mirror. The current mirror may also be realized in many different ways, for example, with use of pnp transistors.
Transistors M7 and M8, indicated by reference numerals 46 and 48, respectively, comprise n-channel MOSFET transistors connected as a current mirror. The current mirror may also be realized in many different ways, for example, with use of npn transistors.
In known fault detector circuits, when the maximum voltage which may appear at the drain and source of the high current output switch transistor is relatively high, resistance voltage dividers must be used to attenuate these voltages to levels which can be accommodated by low cost, low complexity voltage comparators.
In addition, some fault detection applications may require that very low values of current be used to detect open load faults. However, the current drawn by the voltage divider resistors in existing fault detectors undesirably increases the level of such a detect current.
Accordingly, there is a need for an improved fault detector circuit and method for overcoming the problems in the art as discussed above.
The use of the same reference symbols in different drawings indicates similar or identical items. Skilled artisans will also appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.